Context. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. Aims. We wish to ...understand if there are differences between the BL Lac objects that have been detected with the current-generation TeV instruments and those objects that have not yet been detected. Methods. We used a maximum-likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculated the time derivative of the electric vector position angle (EVPA) change. We also studied the spread in the Stokes Q/I−U/I plane and rotations in the polarization plane. Results. The mean polarization fraction of the TeV-detected BL Lacs is 5%, while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q/I−U/I plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seem to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.
We present the characterization of two engineered diffusers mounted on the 2.5-meter Nordic Optical Telescope, located at Roque de Los Muchachos, Spain. To assess the reliability and the efficiency ...of the diffusers, we carried out several test observations of two photometric standard stars, along with observations of one primary transit observation of TrES-3b in the red (R band), one of CoRoT-1b in the blue (B band), and three secondary eclipses of WASP-12b (V band). The achieved photometric precision is in all cases within the submillimagnitude level for exposures between 25 and 180 s. Along with a detailed analysis of the functionality of the diffusers, we add a new transit depth measurement in the blue (B band) to the already observed transmission spectrum of CoRoT-1b, disfavoring a Rayleigh slope. We also report variability of the eclipse depth of WASP-12b in the V band. For the WASP-12b secondary eclipses, we observe a secondary depth deviation of about 5σ, and a difference of 6σ and 2.5σ when compared to the values reported by other authors in a similar wavelength range determined from Hubble Space Telescope data. We further speculate about the potential physical processes or causes responsible for this observed variability.
Detection of the host galaxy of S5 0716+714 Nilsson, K.; Pursimo, T.; Sillanpää, A. ...
Astronomy and astrophysics (Berlin),
08/2008, Letnik:
487, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We have acquired a deep i-band image of the BL Lacertae object S5 0716+714 while the target was in an low optical state. Due to the faintness of the nucleus, we were able to detect the underlying ...host galaxy. The host galaxy is measured to have an I-band magnitude of $17.5 \pm 0.5$ and an effective radius of ($2.7 \pm 0.8$) arcsec. Using the host galaxy as a “standard candle”, we derive $z = 0.31 \pm 0.08$ (1σ error) for the host galaxy of S5 0716+714. This redshift is consistent with the redshift $z = 0.26$ determined by spectroscopy for 3 galaxies close to S5 0716+714. The effective radius at $z = 0.31$ would be $12 \pm 4$ kpc, which is consistent with values obtained for BL Lac host galaxies. An optical spectrum acquired during the same epoch shows no identifiable spectral lines.
We report the serendipitous discovery of a bright point source flare in the Abell cluster A1795 with archival EUVE and Chandra observations. Assuming the EUVE emission is associated with the Chandra ...source, the X-ray 0.5-7 keV flux declined by a factor of ~2300 over a time span of 6 yr, following a power-law decay with index ~2.44 + or - 0.40. The Chandra data alone vary by a factor of ~20. The spectrum is well fit by a blackbody with a constant temperature of kT ~ 0.09 keV (~10 super(6) K). The flare is spatially coincident with the nuclear region of a faint, inactive galaxy with a photometric redshift consistent at the 1sigma level with the cluster (z = 0.062476). We argue that these properties are indicative of a tidal disruption of a star by a black hole (BH) with log(M sub(BH)/M sub(middot in circle)) ~ 5.5 + or - 0.5. If so, such a discovery indicates that tidal disruption flares may be used to probe BHs in the intermediate mass range, which are very difficult to study by other means.
ABSTRACT
Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in ...2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.
High-redshift (z > 2) blazars are the most powerful members of the blazar family. Yet, only a handful of them have both X-ray and γ-ray detection, thereby making it difficult to characterize the ...energetics of the most luminous jets. Here, we report, for the first time, the Fermi-Large Area Telescope detection of the significant γ-ray emission from the high-redshift blazar DA 193 (z = 2.363). Its time-averaged γ-ray spectrum is soft (γ-ray photon index = 2.9 0.1), and together with a relatively flat hard X-ray spectrum (14-195 keV photon index = 1.5 0.4), DA 193 presents a case to study a typical high-redshift blazar with inverse Compton peak being located at MeV energies. An intense GeV flare was observed from this object in the first week of 2018 January, a phenomenon rarely observed from high-redshift sources. What makes this event a rare one is the observation of an extremely hard γ-ray spectrum (photon index = 1.7 0.2), which is somewhat unexpected because high-redshift blazars typically exhibit a steep falling spectrum at GeV energies. The results of our multifrequency campaign, including both space- (Fermi, NuSTAR, and Swift) and ground-based (Steward and Nordic Optical Telescope) observatories are presented, and this peculiar γ-ray flare is studied within the framework of a single-zone leptonic emission scenario.
ABSTRACT
Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4 0954+65 with data acquired in 2019–2020 by the Transiting ...Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37 GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time.
Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales
. This variability seems mostly random, although some ...quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare
in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet
, plasma instabilities
or orbital motion in an accretion disc
. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref.
). BL Lac, the prototype of a subclass of blazars
, is powered by a 1.7 × 10
M
(ref.
) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref.
)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities
near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.
Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory. ...Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ 287. This quasar shows quasi-periodic optical outbursts at 12-year intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened 20 days later.
Abstract
We report results from a one-week multiwavelength campaign to monitor the BL Lacertae object (BL Lac) S5 0716+714 (on 2009 December 9-16). Nine ground-based telescopes at widely separated ...longitudes and one space-based telescope aboard the Swift satellite collected optical data. Radio data were obtained from the Effelsberg and Urumqi observatories and X-ray data from Swift. In the radio bands, the source shows rapid ∼(0.5-1.5) d intraday variability with peak amplitudes of up to ∼10 per cent. The variability at 2.8 cm leads by about 1 d the variability at 6 and 11 cm. This time lag and more rapid variations suggest an intrinsic contribution to the source's intraday variability at 2.8 cm, while at 6 and 11 cm, interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ∼0.8 mag were detected in the V, R and I bands. The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 d time-scale, favouring reprocessed inverse Compton over synchrotron radiation in this band. The characteristic variability time-scales in radio and optical bands are similar. A quasi-periodic variation of 0.9-1.1 d in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical bands are discussed. The lack of a strong radio-optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical) and is consistent with a high jet opacity and a compact synchrotron component peaking at ≃100 GHz in an ongoing very prominent flux-density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution, including γ-ray data from the Fermi satellite. We obtain lower limits for the relativistic Doppler boosting of δ ≥ 12-26, which for a BL Lac-type object is remarkably high.
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